Method and device for driving bore-holes, in the sea bed using a counterflush method

ABSTRACT

Method of, and apparatus for, sinking bore holes, in particular exploratory and extraction bore holes, in the sea bed ( 40 ). The material from the sea bed is loosened with the aid of a rotating drill head ( 10 ) which is arranged at the bottom end of a drill string ( 5 ) which is suitable for transmitting a torque. The top end of the drill string is mounted on a floating platform ( 1 ), it being the case that the drill string ( 5 ) comprises at least two telescopically interengaging drill-string parts ( 6, 7 ) which, during a vertical movement of the platform, induced for example by the sea swell, execute a movement relative to one another, with the result that the drill head (10) rests with an essentially constant force on the surface which is to be cleared away.

BACKGROUND OF THE INVENTION

The invention relates to a method of, and an apparatus for, sinking boreholes, in particular exploratory and extraction bore holes.

Exploratory bore holes are intended for the purpose of examiningdeposits and to make it possible to take samples of the material presentin the deposit. Exploratory bore holes are sunk, in particular, when thedeposit is at a considerable depth and/or bodies of water lying abovethe deposit, such as seas or oceans, preclude the possibility of sinkingexploratory shafts.

Extraction bore holes serve the purpose of loosening deposit contentsfrom soil strata. An example for extraction bore holes which may bementioned is the extraction of marine sediments with diamond inclusionsdeposited on the sea bed. Such diamond-containing deposits have usuallyformed upstream of estuaries in the form of reasonably sized layersspread out over a rocky base. For extracting the diamond-containingsediment material, use is usually made of apparatuses which comprise adrill head which is lowered to the sea bed with the aid of an extensibledrill-pipe string extending from a floating platform. Floating platformis to be understood as any arrangement of which the height above the seabottom or ocean floor depends on the water level at any one time.Drilling vessels are particularly suitable here.

In order to make the drill head rotate, this rotational movement beingrequired for the drilling operation, either said drill head is arrangedin a rotationally fixed manner on the drill-pipe string and the latteris made to rotate with the aid of a rotary drive (powered rotary head)arranged above the platform, or the drill string is mounted in arotationally fixed manner on the platform and there is provided a drivewhich makes the drill head rotate with respect to the drill string.

The overburden which is loosened during the drilling operation isusually delivered up to the platform, through the interior of the drillstring, by so-called “reverse circulation” —for example by using theknown air-lift method—and at the platform is fed, via a pipe bend whichis provided at the top end of the drill string and is in connection withthe inner volume thereof, to arrangements in which diamonds andoverburden are separated in a known manner.

Since the prerequisite for a satisfactory drilling result is that thedrill-head end side which is equipped with at least one cutting toolalways butts throughout the drilling operation, with an approximatelyconstant advancement pressure against the end side of the bore hole, theplatform comprises an arrangement which allows a vertical movement ofthe platform, called for example by sea swell, without this resulting inthe advancement force fluctuating to any great extent or in the drillhead being lifted off from the sea bed at all. Such arrangementscomprise pneumatic or hydraulic/pneumatic piston/cylinder arrangementswhich are usually connected to a relatively large equalizing volume andare connected, for example, on the cylinder side to the drill stringand, on the piston side, to an arrangement which bears the drill string,for example a drilling derrick. The equalizing volume connected to thecylinders means that the platform can execute vertical movements withoutthe force with which the drilling tool butts against the sea bedchanging to any considerable extent. If the fastening point of thepiston/cylinder arrangement on the platform side is located beneaththose on the drill string, then it is possible, by adjusting thepressure in the equalizing reservoirs, for the contact pressure of thedrill head on the sea bed to be adjusted to a desired value from amaximum of the resultant weight.

Although such arrangements for equalizing vertical movements of theplatform have frequently been used for some time now, it isdisadvantageous that the drill string has to project out of the top sideof the platform at least by half the maximum expected displacement ofthe vertical movement of the platform—in relation to the centralposition thereof—and it is also only at this height that it is possibleto fit the pipe bend which is necessary for discharging the overburdento the separating arrangement. This is disadvantageous because, on theone hand, the center of gravity of the platform is displaced upward to aconsiderable extent, as a result of which the period in which theweather is favorable enough for use of the platform is reduced, and, onthe other hand, the overburden has to be delivered up to the level ofthe pipe bend. This is associated with high energy outlay, inparticular, when the platform is located at its lowest level, forexample in a wave trough, since the energy outlay depends on the heightof the pipe bend above the water surface.

U.S. Pat. No. 3,319,726 discloses an apparatus which is intended forsinking bore holes in the sea bed and in the case of which, in order toequalize the vertical movement of the platform, the drill stringcomprises two telescopically interengaging drill-string parts which canbe displaced axially relative to one another. Serving for mounting theinner and outer drill-string parts is a high-outlay arrangement whichcomprises a plurality of seals and floating pistons for pressureequalization, is filled with a hydraulic fluid and is fully encapsulatedtoward the outside against the penetration of sediment. The disadvantagewith this apparatus is that it involves high outlay to produce.

Furthermore, U.S. Pat. No. 3,259,198 and U.S. Pat. No. 3,669,993disclose apparatuses in the case of which the drill string comprises atelescopic part which is intended to equalize, inter alia, the movementinduced by the sea swell. In the case of these apparatuses, the drilledmaternal is delivered upward via the interior of the drill string byfresh water being fed to the bore hole under pressure. For this purpose,the drill string contains two concentrically arranged tubes which forman annular gap between them. The fresh water is fed by way of theannular gap, and the drilled material is transported away by way of theinner tube. The drill strings of these apparatuses too involve higheroutlay to produce.

SUMMARY OF THE INVENTION

The object of the invention is thus further to develop a method and anapparatus of the abovementioned type so as to improve on thesedisadvantages.

This object is achieved by an embodiment of the method and by anembodiment of the apparatus set out below.

Since, in the case of the method according to the invention, the drillstring comprises at least two telescopically interengaging drill-stringparts which, during a vertical movement of the platform, induced forexample by the sea swell, execute a movement relative to one another, itis no longer necessary for the drill string to project beyond theplatform by at least half the maximum expected vertical deflection ofthe platform; rather, it is possible for the top end of the drill stringto be located directly above the platform surface. As a result, on theone hand, this considerably reduces the height of the center of gravityof the platform, with the result that, in relation to those platforms inthe case of which the vertical movement is equalized above the platform,it is also possible to use this platform in rough seas, and, on theother hand, the now likewise lower pipe bend considerably reduces thedelivery height which is necessary for lifting the overburden, with theresult that, with the energy requirement remaining the same, thedelivery quantity can be increased or else, with the delivery quantityremaining the same, the energy requirement can be lowered. The negativepressure which prevails in the interior of the drill string when use ismade of a reverse circulation method, preferably the air-lift method,which is know per se from FR 2523205 for example, means that a certainquantity of surrounding water is always sucked in through the annulargap which is inevitably located between the telescopically interengagingdrill-string parts, so that there is always water circulating aroundthis region, as a result of which any sediment parts which may havepenetrated are always washed out. It is thus possible to dispense withseals and specially protected mounting arrangements.

In the case of one possible embodiment of the method according to theinvention, the drill head is arranged in a rotationally fixed manner onthe drill string. In order to produce the rotation of the drill head,there is provided on the platform a drive which makes the drill stringrotate.

In the case of one of the preferred embodiments of the method, however,the drill string is mounted in a rotationally fixed manner on theplatform and the drill head is made to rotate relative to the drillstring. By virtue of this configuration, on the one hand, the drivewhich increases the weight of the platform and possibly displaces itscenter of gravity upward is rendered superfluous and, on the other hand,it is possible for the drill string to serve for routing electric,hydraulic or similar lines without high-outlay rotary lead-throughs orcouplings being required for this purpose. It is then also easilypossible for one end of a cable which is routed to the platform to befitted on the bottom drill-string part, as a result of which it ispossible, without high energy outlay, for the bottom drill-string part,together with the drill head, to be lifted, for example, from the bottomof the bore hole and displaced over the sea bed to the location at whicha further bore hole is to be sunk.

The apparatus-related aspect of the invention is one preferredembodiment.

In the case of on of the preferred embodiments, the drill stringcomprises an outer and an inner drill-string part, it being the casethat—in order that the vertical movement of the platform can beequalized—the inner drill-string part is pushed into the outerdrill-string part in the normal position of the platform, at least overa length which corresponds to the maximum upward vertical movements ofthe platform which are to be expected as a result of sea swell or thelike, and can be displaced further into the outer drill-string part, atleast by an amount which corresponds to the maximum expected downwarddeflection of the platform out of its normal position.

Since the inner and outer drill-string parts have to be connected to oneanother in a rotationally fixed manner, in order to be able to transmitthe drive torque or the reaction torque, a configuration of the drillstring including rails and sliding elements is recommended. This ensuresthat, despite the fact that they are fixed rotationally, a low-frictionmovement of the two drill-string parts relative to one another ispossible.

A particularly preferred technical configuration of the mounting whichworks on this principle forms the subject matter of other preferredembodiments as set out below.

It is possible, on the one hand, for the outer drill-string part to bearranged on the platform by way of its top end and for the drill head tobe arranged at the bottom end of the inner drill-string part. In thecase of one of the preferred embodiments, however, the arrangement isthe other way round, since the top, preferably longer top drill-stringpart, which during the drilling operation, in a vertical direction, isconnected rigidly to the platform, comprises components which involveless outlay to produce.

In the case of a first possible embodiment of the apparatus according tothe invention, there is provided on the platform an arrangement withwhich the drill string can be provided to rotate about its longitudinalaxis. In this case, the drill head is arranged in a rotationally fixedmanner on the drill string.

In the case of one of the preferred embodiments. however, the drillstring is mounted in a rotationally fixed manner on the platform andthere is provided a rotary drive, with the aid of which the drill headcan be made to rotate with respect to the drill string.

In the case of the above mentioned preferred embodiment the rotationallyfixed mounting of the drill string on the platform preferably takesplace by means of a rotationally fixed, cardanic retaining means(gimbal).

The drill head is preferably driven either electrically orhydraulically, it being the case that the drive arrangements arepreferably integrated in the drill head or are arranged directly abovethe same.

The pressure line which is required for using the air-lift method may,according to an alternative embodiment, be secured on the outercircumference of the drill string and run parallel to the longitudinalcenter axis thereof.

In the case of an advantageous embodiment, however, the pressure line isdesigned as a pressure hose which can be unrolled from a winding drum.This measure has the advantage that the pressure hose can easily beconnected both to the top drill-string part and to the bottomdrill-string part.

An advantageous development of the apparatus according to the inventionforms the subject matter of an alternative embodiment. In this case, atleast one buoyancy element is arranged on the drill-string part whichbears the drill head. In the case of the apparatus according to theinvention, selection of the buoyancy volume of the buoyancy elementmakes it possible to adjust the desired contact pressure of the drillhead on the sea bed.

In order to avoid the situation where the bottom drill-string part issubjected to compressive loading by too high a buoyancy volume of thebuoyancy element, it is advantageous, according to an alternativeembodiment, for the at least one buoyancy element to be arranged in thevicinity of the top end of the bottom drill-string part.

Straightforward adjustment of the contact pressure, this adjustment alsobeing possible once the drill head has been lowered onto the sea bed, isprovided if the buoyancy elements are designed as floodable tanks whichcan be filled with compressed air as desired, via correspondingcompressed-air lines, possibly by a compressor located on the platform.

If, according to an alternative embodiment, a buoyancy element is alsoprovided on the drill-string part which is mounted on the platform, itis possible to compensate for some of the weight acting on the platform,with the result that heavier, and thus longer, drill-string parts can besecured and the apparatus is also suitable for sinking exploratory boreholes at considerable depth.

BRIEF DESCRIPTION OF THE DRAWINGS

The method according to the invention and an exemplary embodiment of anapparatus according to the invention are illustrated in the drawings, inwhich:

FIG. 1 shows a side view of an embodiment of the apparatus according tothe invention in the case of which the drill string has already beenassembled to its full length, but has not yet been lowered into itsoperating position;

FIG. 2 shows the same embodiment with a drill string which is located inits operating position on the platform but has the drill head lifted offfrom the sea bed;

FIG. 3 shows the same embodiment at the end of a drilling operation;

FIG. 4 shows an enlarged illustration of the detail IV in FIG. 3;

FIG. 5 shows a cross section along section line V—V in FIG. 4;

FIG. 6 shows an illustration, in longitudinal section and in detailform, of the drill string in the region in which the inner drill-stringpart projects into the outer drill-string part.

FIG. 7 shows a schematic view of an alternative embodiment of theinvention in which a powered rotary head is arranged on the platform forrotating the drill string, and the drill head is arranged in arotationally fixed manner on the drill string.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The apparatus, which is designated as a whole by 100 in FIGS. 1 to 3,comprises a mast 2 which is arranged on a floating platform 1 and isonly indicated in FIGS. 1 to 3. As is likewise merely indicated in FIG.1, it is equipped with a block and tackle 3 which serves for lifting orlowering one or more segments 4, 4′ of a drill string, which isdesignated as a whole by 5.

Serving for providing and transferring the drill-string segments 4, 4′to the block and tackle 3 are means which are known for this purpose andare usually referred to as pipe erector or pipe handling system, but arenot illustrated in the drawing.

The drill string 5, which—as has already been explained in theintroduction—comprises removable segments 4, 4′, has a top drill-stringpart 6 and a bottom drill-string part 7. The top drill-string part 6opens out telescopically into the bottom drill-string part 7 at thelocation 8 and, according to the illustration in FIG. 1, projects intosaid bottom drill-string part approximately as far as location 9. Thetop and the bottom drill-string parts 6, 7 are configured in that lengthregion which is provided for the insertion of the top drill-string part,such that, in this length region, the drill-string parts 6, 7 can movewith low friction relative to one another in the longitudinal directionof the drill string, although rotation of the two drill-string parts 6,7 with respect to one another about the longitudinal center axis of thedrill string is not possible. A type of mounting which exhibits thesefunctional features is described in more detail hereinbelow withreference to FIGS. 4 and 5.

Arranged at the bottom end of the bottom drill-string part is a drillhead 10 which, with the aid of a rotary drive 11 arranged above it onthe bottom drill-string part, can be rotated relative to the drillstring 5, which in the exemplary embodiment is mounted in a rotationallyfixed manner in the platform and absorbs the reaction torque. In theexemplary embodiment illustrated, the power source used is a hydraulicmotor which is supplied with hydraulic fluid under pressure via ahydraulic line 12. However, it is likewise possible to use an electricdrive instead of the hydraulic drive and to provide an electric lineinstead of the hydraulic line 12.

An alternative embodiment, as shown schematically in FIG. 7, ischaracterized in that arranged on the platform (1) is a powered rotaryhead (81) for making the drill string (5) rotate about its longitudinalaxis, and the drill head (10) is arranged in a rotationally fixed manneron the drill string.

Provided at the top end of the bottom drill-string part are two eyeelements 13 on which there are fastened two cables 16′ which run throughan opening 14, provided in the platform and through which the drillstring 5 also extends, and are fed to a winch 16 via deflection rollers15. The bottom drill-string part can thus be lifted and lowered byvirtue of the winch 16 being actuated.

First of all the basic functioning of the apparatus according to theinvention is to be described hereinbelow with reference to FIGS. 1 to 3.

In the phase which is illustrated in FIG. 1, the drill string 5 hasalready been assembled to its full length by the screw-connection ofindividual segments 4 and 4′. Arranged at the top end of the drillstring is the pipe bend serving for the discharge of lifted overburden.By virtue of the block and tackle 3 and the winch 16 being paid outsynchronously, the drill-string parts 6, 7 are lowered until the topregion 18 of the top drill-string part 6 is located level with acardanically mounted retaining means 19 (gimbal), which is illustratedschematically in the open state in FIG. 1.

The top drill-string part is fixed in this position by virtue of theretaining means 19 being closed. This state is illustrated in FIG. 2.The pipe bend 17 then opens out into an inlet 20 which is widened in theform of a funnel at its end and feeds the overburden to a knownarrangement (not illustrated in the drawing) for separating off diamondscontained in the overburden.

In the state which is illustrated in FIG. 2, the apparatus 100 ispositioned such that the drill head is located above that part of thesea bed which is to be cleared away.

Before the actual drilling operation begins, the winch 16 is paid out,as a result of which the bottom drill-string part 7 is lowered further,by sliding down on that part of the top drill-string part 6 whichprojects into it, until the drill head 10 rests on the sea bed. Thecables 16′ are then paid out further, with the result that—as isillustrated in FIG. 3—they hang down in a slack loop. The resultantweight of the bottom drill-string part 7 and of the components connectedthereto determines the contact pressure between the end side of thedrill head and the sea bed. During further advancement of the drill head10 into the sea bed 40, the bottom drill-string part 7 can slide downfurther without obstruction from the top drill-string part 6.

If the platform—induced for example by the sea swell—executes a verticalmovement, as is in tended to be symbolized by the double arrow in FIG.3, it being possible for this vertical movement to measure quite a fewmeters, depending on the weather conditions then the contact pressure ofthe drill head 10 on the sea bed 40 is not affected as a result sincethe inner drill-string part 6 can be displaced with low friction in andout of the bottom drill-string part 7 corresponding to the verticalmovement of the platform.

In order for it to be possible to adjust the end-side contact pressureof the drill head 10 on the sea bed 40, a buoyancy unit 21 is arrangedin the region of the top end of the bottom drill-string part. Thisbuoyancy unit comprises a plurality of tanks (not shown in the drawing)which may optionally be flooded or, with the aid of over compressed-airlines (not illustrated in the drawing) emptied, with the result that thebuoyancy can be adjusted and the resultant contact pressure between thedrill head and the sea bed can thus be reduced to the value required ineach individual case.

A preferred technical configuration of the mounting between the top andthe bottom drill-string parts 6, 7 is to be explained hereinbelow withreference to FIGS. 4 and 5.

The top drill-string part, which is of essentially tubularconfiguration, comprises a tubular part 23 which has a round crosssection and is subdivided into segments 4 which can be screw-connectedto one another by means of flanges. Over its length which can beintroduced into the bottom drill-string part, four angle profiles 22,which are distributed uniformly over the circumference, are arranged onthe tubular part 23 parallel to the longitudinal center axis L. Eachangle profile 22 comprises two legs 22′, 22″ which are at right angleswith respect to one another. The angle profiles 22 are welded over theirlength—in any case in certain sections thereof—to the tubular part 23 ofthe top drill-string part 6 by way of the free edges of the legs 22′,22″.

Serving for mounting the top drill-string part 6 in the bottomdrill-string part 7, said mounting allowing the top and bottomdrill-string parts to move relative to one another in the axialdirection and being suitable for transmitting torques directed about thelongitudinal center axis L, are roller arrangements 24 which arearranged on the bottom drill-string part 7, are distributed over thecircumference, corresponding to the angle profile 22, and are inoperative connection with the angle profiles. Each roller arrangement 24comprises two pairs of rollers 25, 26, it being the case that therollers 25, 25′; 26, 26′ belonging to a pair are spaced apart from oneanother in the direction of the longitudinal center axis and have theiraxes of rotation running parallel to one another, whereas the axes ofrotation of the roller pairs 25, 26 of a roller arrangement 24 arealigned perpendicularly to one another. In the case of the exemplaryembodiment which is illustrated in FIG. 4, two sets of in each case fourroller arrangements 24, 24′ are provided in the longitudinal directionof the drill string in order for it to be possible to prevent canting ofthe top drill-string part in the bottom drill-string part. However, itis likewise possible to provide a greater number of roller pairs.

Each roller arrangement 24 comprises a roller holder 27 which isarranged on the outer lateral surface of an essentially tubular segment4′ of the bottom drill-string part 7. Bores for receiving roller shafts29 are made in the roller holder 27 corresponding to the alignment andnumber of rollers 28 belonging to a roller arrangement 24. In this case,the bores are arranged such that the axes of rotation S are locatedessentially outside the cross section of the bottom drill-string part 7,but the rollers 28 project, through openings 30, into the inner crosssection of the segment 4′, with the result that the running surfaces 31of the rollers are aligned in a manner corresponding to the outersurfaces of the angle profiles 22 and, when the drill-string parts 6, 7have been pushed one inside the other, roll on the outer surfaces of theangle profiles.

This type of mounting ensures low-friction displaceability of the topand of the bottom drill-string parts in the longitudinal directionrelative to one another, while at the same time it is possible totransmit high reaction torques about the longitudinal center axis L.

The overburden is delivered from the bottom of the bore hole to theplatform by the known air-lift method (not illustrated in the drawing),in the case of which air is blown into the interior of the drill stringvia a corresponding feed line or pressure line PL, and air inlet Al.

In order to prevent the sediment which passes into the drill string frompenetrating into the mounting between the top and bottom drill-stringparts 6, 7 and obstructing the capacity of the parts for low-frictionmovement relative to one another, an inner tube 36 (“rapier tube”) isflanged—as can be seen from FIG. 6—at the bottom end of the topdrill-string part, which projects into the bottom drill-string part 7,said inner tube projecting into that part of the bottom drill-stringpart 7 which is located beneath it and terminating in an open state justabove the rotary drive 11. The bottom drill-string part 7 is ofdouble-walled design in this region, the inner wall 47 being formed byan inner tube 48, of which the internal diameter is dimensioned suchthat a narrow annular gap 49 is formed between said internal diameterand the external diameter of the inner tube 46.

By virtue of this measure, the loosened sediment penetrates into theinterior of the top drill-string part through the bottom opening of theinner tube 36, as a result of the negative pressure which prevails inthe inner volume of the top drill-string part by virtue of the air-liftmethod being used, with the result that even at this stage said sedimentcannot come into contact with the roller arrangements 24 or angleprofiles 22. Furthermore, the negative pressure which prevails in theinterior of the top drill-string part 6 means that there is always acertain quantity of surrounding water which is sucked in through theannular gap 49, from the top end of the bottom drill-string part 7 andcirculates around the roller arrangement 24 and the angle profiles 22,with the result that any fractions of sediment which may have penetratedare always washed out.

What is claimed is:
 1. Apparatus for sinking bore holes, in the sea bed,having a drill string which is suitable for transmitting a torque and ofwhich the top end is mounted on a floating platform and of which thebottom end bears a rotationally driveable drill head and which comprisesa plurality of drill-string parts which can be displaced with lowfriction in the axial direction of the drill string relative to oneanother, characterized in that there is provided an air inlet which isin connection with the drill-string interior and through which it ispossible to blow in compressed air fed via a pressure line;characterized in that the drill string comprises an outer and an innerdrill-string part, wherein the inner drill-string part engages in theouter drill-string part, and wherein the outer drill-string partcomprises an inner tube and an outer tube defining an annular gapbetween an internal diameter of the outer tube and an external diameterof the inner tube.
 2. Apparatus according to claim 1, characterized inthat the drill string comprises an outer and an inner drill-string part,it being the case that the inner drill-string part engages in the outerdrill-string part, at least over a length which corresponds to themaximum upward vertical movements of the platform which are to beexpected as a result of sea swell, and can be displaced further into theouter drill-string part, at least by an amount which corresponds to themaximum expected downward deflection of the platform out of its normalposition.
 3. Apparatus according to claim 2, characterized in that theinner drill-string part comprises, at least over the maximum length bywhich it projects into the outer drill-string part, rails, which aredistributed over its outer circumference and on which there run slidingelements, which are distributed correspondingly over the innercircumference of the outer drill-string part.
 4. Apparatus according toclaim 2, characterized in that the inner drill-string part is mounted onthe platform by way of its top end, and the outer drill-string partbears the drill head at its bottom end.
 5. Apparatus according to claim2, characterized in that arranged on the platform is a powered rotaryhead for making the drill string rotate about its longitudinal axis, andthe drill head is arranged in a rotationally fixed manner on the drillstring.
 6. Apparatus according to claim 2, characterized in that thedrill string is mounted in a rotationally fixed manner on the platformand there is provided a rotary drive, with the aid of which the drillhead can be made to rotate with respect to the drill string. 7.Apparatus according to claim 6, characterized in that the rotary driveis operated electrically.
 8. Apparatus according to claim 6,characterized in that the rotary drive is operated hydraulically. 9.Apparatus according to claim 1, characterized in that the pressure linefor blowing the compressed air into the interior of the drill string issecured on the outer circumference of the drill string and runs parallelto the longitudinal center axis thereof.
 10. Apparatus according toclaim 1, characterized in that the pressure line provided is a pressurehose which can be unrolled from a winding drum.
 11. Apparatus accordingto claim 1, characterized in that at least one buoyancy element isprovided on the drill-string part of which the top end is mounted on theplatform.
 12. Apparatus for sinking bore holes, in the sea bed, having adrill string which is suitable for transmitting a torque and of whichthe top end is mounted on a floating platform and of which the bottomend bears a rotationally driveable drill head and which comprises aplurality of drill-string parts which can be displaced with low frictionin the axial direction of the drill string relative to one another,characterized in that there is provided an air inlet which is inconnection with the drill-string interior and through which it ispossible to blow in compressed air fed via a pressure line;characterized in that the drill string comprises an outer and an innerdrill-string part, it being the case that the inner drill-string partengages in the outer drill-string part, at least over a length whichcorresponds to the maximum upward vertical movements of the platformwhich are to be expected as a result of sea swell, and can be displacedfurther into the outer drill-string part, at least by an amount whichcorresponds to the maximum expected downward deflection of the platformout of its normal position; characterized in that the inner drill-stringpart comprises, at least over the maximum length by which it projectsinto the outer drill-string part, rails, which are distributed over itsouter circumference and on which there run sliding elements, which aredistributed correspondingly over the inner circumference of the outerdrill-string part; characterized in that the inner drill-string part isof tubular design and the rails comprise angle profiles which run in thedirection of the longitudinal axis (L) of the inner drill-string partand are welded on the outer surface of the inner drill-string part byway of their free longitudinal edges.
 13. Apparatus according to claim12, characterized in that the outer drill-string part comprises, in thelongitudinal direction of the same, at least one running-roller pair,which is arranged such that the two rollers of the running-roller pairare each assigned to an outer surface of a corresponding one of theangle profiles and are supported with rolling action on the same. 14.Apparatus according to claim 13, characterized in that each of the angleprofiles comprises two legs which are at right angles with respect toone another, and each angle profile is assigned at least twolongitudinally spaced-apart roller pairs each running on one leg outersurface.
 15. Apparatus according to claim 14, characterized in that theouter drill-string part is of tubular design and comprises rollerholders which are fastened on its outer circumference and each receiveat least one roller, such that the axis thereof is located outside thecross section of the outer drill-string part and the running surfacethereof projects, through a cutout made in the outer drill-string part,into the inner cross section of the outer drill-string part. 16.Apparatus for sinking bore holes, in the sea bed, having a drill stringwhich is suitable for transmitting a torque and of which the top end ismounted on a floating platform and of which the bottom end bears arotationally driveable drill head and which comprises a plurality ofdrill-sting parts which can be displaced with low friction in the axialdirection of the drill string relative to one another, characterized inthat there is provided an air inlet which is in connection with thedrill-string interior and through which it is possible to blow incompressed air fed via a pressure line; characterized in that the drillstring comprises an outer and an inner drill-string part, it being thecase that the inner drill-string part engages in the outer drill-stringpart, at least over a length which corresponds to the maximum upwardvertical movements of the platform which are to be expected as a resultof sea swell, and can be displaced further into the outer drill-stringpart, at least by an amount which corresponds to the maximum expecteddownward deflection of the platform out of its normal position; andcharacterized in that the drill string is mounted in a rotationallyfixed manner on the platform and there is provided a rotary drive, withthe aid of which the drill head can be made to rotate with respect tothe drill string; further comprising a gimbal for mounting the drillstring on the platform.
 17. Apparatus for sinking bore holes, in the seabed, having a drill string which is suitable for transmitting a torqueand of which the top end is mounted on a floating platform and of whichthe bottom end bears a rotationally driveable drill head and whichcomprises a plurality of drill-string parts which can be displaced withlow friction in the axial direction of the drill string relative to oneanother, characterized in that there is provided an air inlet which isin connection with the drill-string interior and through which it ispossible to blow in compressed air fed via a pressure line,characterized in that a buoyancy unit is arranged on the drill-stringpart which bears the drill head.
 18. Apparatus according to claim 17,characterized in that the buoyancy unit is arranged in the vicinity ofthe top end of a bottom drill-string part.
 19. Apparatus according toclaim 17, characterized in that the buoyancy unit comprises at least onetank which may be filled with compressed air.
 20. A method of sinkingbore holes in the sea bed, the method comprising: mounting a top end ofa drill string to a floating platform; arranging a rotating drill headat a bottom end of the drill string; assembling the drill stringcomprising at least two telescopically interengaging drill string partswhich, during a vertical movement of the floating platform, execute amovement relative to one another with the result that the drill headrests with an essentially constant force on a surface of the sea bedwhich is to be cleared away wherein the at least two telescopicallyinterengaging drill-string parts comprise an outer and an innerdrill-string part, wherein the outer drill-string part comprises aninner tube and an outer tube defining an annular gap between an internaldiameter of the outer tube and an external diameter of the inner tube;loosening material from the sea bed with the drill head; and deliveringout of the bore hole, through an interior of the drill string, by usingan air-lift method, the material that has been loosened from the seabed.
 21. The method of claim 20, wherein the arranging step includesarranging the drill head in a rotationally fixed manner on the drillstring; and wherein the loosening step includes rotating the drillstring, thus producing rotation of the drill head.
 22. The method ofclaim 20, wherein the mounting step includes mounting the drill stringin a rotationally fixed manner to the platform; and wherein theloosening step includes rotating the drill head relative to the drillstring.